.gamma.-Aminobutyric acid (GABA) is an important inhibitory neurotransmitter. When the concentration of GABA in the brain decreases below a threshold level, seizures and other neurological disorders occur (A. V. Delgado-Escueta et al., Basic Mechanisms of the Epilepsies, Raven Press, New York, 365 (1986)). The appropriate level of GABA at the synaptic cleft can be maintained by the irreversible inactivation of the enzyme GABA-T, which is involved in the degradation of GABA (S. M. Nanavati et al., J. Med Chem., 32, 2413 (1989)).
The biotransformation of .gamma.-aminobutyric acid (GABA) to succinic acid semialdehyde, which is catalyzed by the enzyme GABA-transaminase (GABA-T), is the primary reaction responsible for the catabolism of GABA, an inhibitory neurotransmitter of the central nervous system. It is known that low levels of endogenous GABA are associated with seizure disorders (such as those involved in epilepsy, alcohol withdrawal, or barbiturate withdrawal), with disorders involving involuntary movement (such as those caused by the extrapyrimidal effects of drugs, for example tardive dyskinesia) with certain psychiatric disorders (such as schizophrenia and depression) and with muscle spasticity. Blockade of the transformation of GABA to succinic acid semialdehyde, such as by irreversible inhibition of GABA-T, can elevate GABA levels in the central nervous system (CNS) and, thus provides a means for treating the disorders of the CNS associated with low GABA levels.
Certain compounds are known to be irreversible inhibitors of GABA-T and thereby to elevate brain levels of GABA. Examples are 4-aminohex-5-enoic acid ("vinyl GABA"), 4-aminohex-5-ynoic acid ("acetylenic GABA" or "ethynyl GABA") and 4-amino-hepta-5,6-dienoic acid ("allenyl-GABA") (see U.S. Pat. Nos. 3,960,927, 3,959,356, and 4,454,156; Lippert et al., Eur. J. Biochem., 74, 441 (1977); Lippert et al., Brain Research Bulletin, 5((2), 375 (1980); Jung et al., J. Neurochem., 28, 717 (1977); Palfreyman et al., GABA-Neuro-Transmitter, Alfred Benzon Symposium 12; Larsen et Editors, Munksgaard, Copenhagen, 432-446 (1979); June al., Biochemical and Biophysical Research Comm., 67, 301 (1975); Palfreyman et al., Biochemical Pharm., 30, 817 (1981); and, Jung, et al., Biochemical Pharm., 33, 3717 (1984)).
In particular, these compounds are useful as anticonvulsants for the control of seizures involved in epilepsy. Anticonvulsant activity can be demonstrated by means of standard test procedures in laboratory animals against experimentally-induced seizures. These inhibitors of GABA (.gamma.-ethynyl 1, .gamma.-allenyl 2, and .gamma.-vinyl 3, GABAs) have been designed and synthesized. ##STR1## All these compounds have potential for therapeutic use and .gamma.-vinyl GABA (vigabatrin) has already been approved in Europe as an effective drug for the treatment of epilepsy.
The biological activity of .gamma.-allenyI GABA and .gamma.-vinyl GABA resides in the (S)-enantiomers (P. Casara et al., Tetrahedron Letters, 25, 1891 (1984)). Conversely, (R)-.gamma.-ethynyl GABA is more active as an anticonvulsant agent than its (S)-counterpart or racemic compound (M. J. Jung et al., Biochemistry, 17, 2628 (1978)). So far the enantiomers of .gamma.-ethynyl GABA, .gamma.-allenyl GABA, and .gamma.-vinyl GABA have been produced by asymmetric synthesis (P. Casara et al., and A. Holmes et al., J. Chem. Soc., Perkin Trans. 1, 3301 (1991)) or diastereomer crystallization (M. J. Jung et al., and C. Danzin et al., Chemical and Biological Aspects of Vitamin B6 Catalysis, A. E. Evangepoulos ed., Alan R. Liss, New York, Part A, 377-385 (1984)). These methods, however, are not suitable for large-scale synthesis, since the routes are long and the yield of the final product is low.
Compounds .gamma.-ethynyl GABA, .gamma.-allenyl GABA, and .gamma.-vinyl GABA are difficult targets for enzyme-based resolution techniques as well (C. J. Sih et al., Stereochem., 19, 63-125 (1898), and A. M. Klibanov, Acc. Chem. Res., 23, 114-120 (1990)). Enzymes, such as aminoacylases (H. K. Chenault et al., J. Am. Chem. Soc., 111, 6354-64 (1989)) and amino-peptidase (E. M. Meijer et al., Biocatalysts in Organic Synthesis (eds. J. Tramper et al., Amsterdam:Elsevier, 135-156 (1985)), that are normally used for the resolution of .alpha.-amino acids cannot resolve .gamma.-amino acids. Lipases catalyze the enantioselective hydrolysis of the esters of (N-acyl)-.gamma.-vinyl GABA, but with modest stereoselectivity. These compounds may also present a serious problem for a newly developed technique with .omega.-amino acid transaminases (D. I. Stirling et al., U.S. Pat. No. 4,950,606 (1990)), since .gamma.-ethynyl GABA, .gamma.-allenyl GABA, and .gamma.-vinyl GABA, are designed to irreversibly inhibit the very same group of enzymes.
Here we report a simple procedure for the preparation of the enantiomers of .gamma.-ethynyl GABA, .gamma.-allenyl GABA, and .gamma.-vinyl GABA by penicillin acylase-catalyzed hydrolysis of the corresponding N-phenylacetyl derivatives. Penicillin acylase (PA) from E.coli is used in industry for the preparation of 6-aminopenicillanic acid and semisynthetic .beta.-lactam antibiotics (V. K. Svedas et al., Enzyme Microb. Technol., 2, 138 (1980)). PA is highly specific to phenylacetyl group and catalyzes its cleavage not only from penicillins, but also from amides, peptides, and esters (M. Cole, Biochem J., 115, 733 (1969); Ibid, 741; and, A. Czentirmai, Acta Microbiol. Acad. Sci. Hung., 12, 395 (1965/1966). The structure of the leaving group of the substrates hardly affects the rate constants of the hydrolytic reactions (M. Cole, Nature, 203, 519 (1964), and A. L. Margolin et al., Biochim. Biophys. Acta, 616, 283 (1980)). The enantioselectivity of PA was exploited in the preparation of amino acids (D. Rossi et al., Experientia, 33, 1557 (1977) and Ibid, 41, 35 (1985)), aminoalkylphosphonic acids (V. A. Solodenko et al., Tetrahedron, 47, 3989 (1991)), esters and alcohols (C. Fuganti et al., Tetrahedron Letters, 44, 2575 (1988), and H. Waldman, 30, 3057 (1989)), although the hydrolysis of an ester bond normally results in products with modest optical purity. Recently, the high enantioselectivity of PA in the acylation reaction was demonstrated in the synthesis of a new carbacephalosporin, locarbef (M. Zmijewski et al., Tetrahedron Letters, 32, 1621 (1991)).